THE ECHINOIDEA 283 



The very varied habits of the Echinoids further increase the 

 value of this class. Some sea-urchins burrow into sand, and others 

 bore into rocks. Some seek the shelter of rock-pools or the quiet 

 of a muddy sea-floor below the limit of tidal action. Others 

 cling to rocks between tide-lines, choosing the positions that are 

 most exposed to the buffeting of a tropical surf ; others again 

 crawl over, or lie half -buried in, the ooze of abyssal oceanjc 

 depths. Some feed on algae ; others swallow mud and live 

 on the organic matter it contains. Of some the young develop 

 directly, of others indirectly, the latter undergoing a metamor- 

 phosis during development. The modifications in structure by 

 which Echinoids are able to adapt themselves to these different 

 habits are so well marked, that the conditions under which fossil 

 sea-urchins lived can generally be determined. Hence the group is 

 of great value to the geologist. Such rich series of Echinoid faunas 

 are known, that the life-history of the class can be written with 

 greater completeness than that of any other group of Echinoderms, 

 and as completely as that of any class in the animal kingdom. 



Little, however, is certainly known as to the relations of the 

 Echinoidea to other Echinoderms. The group is so compact and 

 well marked, that the distinction between it and the other classes 

 was known to Aristotle, and has never been in doubt. Moreover, 

 the class has been as well defined as it now is since Silurian times. 

 The recognition of the sea-urchins as a distinct group of Echino- 

 derms is, therefore, as old as zoology. A sketch of the history of 

 work upon the Echinoids need only consider the determination of 

 the main points in their anatomy, life-history, and classification. 



The earliest account of the natural history of the sea-urchins 

 is in Aristotle's History of Animals, wherein the common edible 

 species of the Mediterranean (Echinus esculentus) is described 

 with considerable accuracy. Aristotle called this animal Echinus 

 (ExVos), the Greek for hedgehog, a term subsequently given to the 

 best known genus, and used as the root of the name for the class. 

 In the same book three other types of sea-urchins are mentioned, 

 viz. Brissus, Spatangiis, and Echinometra ; and these names are 

 still used in Echinoid nomenclature. Aristotle's account shows 

 that he had studied both the habits and anatomy of the animals ; 

 thus he knew that Echinus could walk upon the tip of its spines, 

 had five teeth, five unpaired ovaries, a pharynx, and stomach. In 

 mediaeval times Echinoids were described by Eondelet (1554) and 

 Aldrovandus (1606 and 1648); but it was not till the beginning 

 of the eighteenth century that any observations of scientific value 

 were published. The first important post-classical contributions 

 to the subject were the works of Breynius (1732) and Klein 

 (1734). The former in his De Echinis et Echinitis, and the 

 latter in his Natumlis Dispositio Echinodermatum, figured and 



